The present invention relates to a method and device for introducing, under pressure carburetted mixture into a cylinder of a Z-stroke engine at an end of an air scavenging, with the pressure source being the pressure existing in a crankcase of the cylinder which is retarded by 120.degree. of the crankshaft (case of a 3, 6, . . . , 3n cylinders engine) or by the crankcase of the cylinder which is retarded by 90.degree. of the crankshaft (case of a 4, 8, . . . , 4n cylinders engine) with respect to the considered cylinder where introduction of a carburetted mixture takes place. This pressure source is not stored.
The introduction of carburetted mixture under pressure takes place at the arrival of gas coming from the pressure source into the considered cylinder during the end of the scavenging phase thereof. The arrival of gas from this pressure source into a fuel metering device prepares a carburetted mixture which may be introduced into the cylinder through an orifice which may be located in the cylinder head and preferably be open solely during the intake of the pressure source.
In this case, the device may comprise a valve controlled so as to open during the arrival of gas coming from this pressure source, or an automatic valve (non-return valve type) whose opening is controlled by the pressure from the pressure source.
The orifice may also be located in the cylinder, and the opening thereof may be controlled by movement of the piston (case of a port) combined with a non-return device of valve type (or rotary cock).
For example, one embodiment of this type may join the crankcase of the cylinder retarded by 120.degree. or 90.degree. of the crankshaft to the considered cylinder via a transfer duct arriving on the side opposite the exhaust into the cylinder considered (duct generally called rear transfer duct).
To the extent that the location for metering the oxydent (upstream of the intake orifice opening into the cylinder) is not under a pressure greater than the ambient pressure for all the time outside the period of carburettor mixture introduction, such dosing may be provided by low pressure injector, but also by means of simple devices such, for example, as a carburetor of the type used in the intake of a two-stroke engine.